Plastisols comprising polymer fine particles dispersed in plasticizers as media are utilized in a wide variety of industrial fields and have very high industrial values. Especially, plastisols prepared using vinyl chloride polymer fine particles are known as vinyl chloride sols and are used in various fields such as wall papers, undercoats for automobiles, body sealers for automobiles, carpet backing materials, floor covering materials and paints due to their excellent properties.
Owing to the unique properties of vinyl chloride polymer fine particles, vinyl chloride sols are markedly excellent in the following basic properties required for plastisols, that is, (1) the polymer particles are not swollen with or dissolved in plasticizer during storage of the plastisols (hereinafter this property being referred to as “storage stability”); and (2) even after plastisols are applied and allowed to gel by heat treatment to form dry coating films, the plasticizer is satisfactorily retained in the dry coating films and does not bleed out with lapse of time (hereinafter this property being referred to as “plasticizer retainability”), and have come to be industrially widely utilized till today.
However, as for products made using vinyl chloride sols, there has been the problem that when they are incinerated, hydrogen chloride gas is generated to seriously damage incinerators. Moreover, recently, there are problems of acid rain and, furthermore, adverse effects on human bodies and earth environment of the highly toxic dioxin generated upon incineration. Therefore, it has been expected to develop a substitutive material which has similar properties to those of vinyl chloride sols while hardly cause environmental problems.
Under the circumstances, one-pack type urethane materials, epoxy materials, aqueous emulsion materials, silicone materials and the like have been proposed as materials substitutable for vinyl chloride sols. However, for the production of these materials, the existing productive facilities for vinyl chloride sols cannot be utilized, and, thus, an enormous facility investment is required for industrial utilization of these materials. Moreover, one-pack type urethane materials have many problems such as insufficient storage stability due to increase of viscosity, toxicity and high cost. The epoxy materials also have problems of high cost and considerably inferior properties to vinyl chloride sols. Problems of the aqueous emulsions are that they cannot be used for thick coating, blisters that occur in the coating films with evaporation of water, and poor water resistance of the coating films. The silicone materials are also high in cost and cannot be a substitute material for vinyl chloride sols from the points of their properties. Therefore, it is considerably difficult to substitute these materials for vinyl chloride sols.
Recently, plastisols comprising acrylic polymer fine particles, namely, acrylic sols, have been proposed as substitutive materials to solve these problems.
For example, JP-A-60-258241, JP-A-61-185518 and JP-A-61-207418 propose novel plastisols obtained by using composites of a vinyl chloride polymer and an acrylic polymer. However, these plastisols containing essentially a vinyl chloride polymer still produce toxic gases upon incineration like the conventional vinyl chloride sols. Therefore, the above-mentioned environmental problems have not yet been solved.
As plastisols containing no vinyl chloride polymers and other halogen-containing polymers, JP-A-5-255563 proposes a plastisol comprising an acrylic polymer. The polymer used in this patent publication comprises particles of uniform structure, and in the case of the acrylic polymer, storage stability of plastisol and plasticizer retainability of the coating film cannot be realized with particles of uniform structure, and, hence, the storage stability of or properties of the coating film formed from the plastisol proposed in the above patent publication tend to be extremely deteriorated.
This is because unlike vinyl chloride polymers, acrylic polymers only have weak van der Waals cohesive force between molecules, and, hence, when a composition high in compatibility with plasticizer is used, the plasticizer readily penetrates between the molecules to cause plasticization, namely, gelation, which lead to deteriorated storage stability.
Therefore, in order to improve storage stability, it is necessary to lower the compatibility with plasticizer. However, a polymer low in compatibility with plasticizer has good storage stability, nevertheless, the coating film obtained by application and heating of the sol (hereinafter referred to as “gelling film”) has considerably low retainability of plasticizer resulting in bleed out of plasticizer from the gelling film with lapse of time.
As explained above, in the case of an acrylic sol prepared using acrylic polymer fine particles, the storage stability and the plasticizer retainability are contrary to each other and polymer particles of uniform structure cannot retain both of them.
JP-A-5-279539 proposes an acrylic plastisol obtained using particles of core-shell structure. In this patent publication, a polymer containing an acid or an acid anhydride is used as the acrylic polymer. However, since the polymer proposed in the patent publication is low in compatibility with plasticizer, and, especially, copolymerization ratio of methyl methacrylate in the shell portion is high, its plasticizing state becomes inferior and it fails to form good coating films when a plasticizer with low polarity such as a phthalic ester plasticizer is used.
In addition, JP-A-6-322225 proposes similarly a plastisol using particles of core-shell structure. According to this technique, the particles of core-shell structure are obtained by preparing particles of uniform structure and thereafter subjecting them to an alkali hydrolysis treatment, thereby converting the ester group in the uppermost surface layer portion of the particles to a carboxyl group. Therefore, the shell portion thereof is very thin and substantially accounts for only about 1% or less of the volume of the particles. Accordingly, the effect of improving storage stability, which is expected as a role of the shell portion, is very low. Furthermore, the shell portion introduced by the alkali hydrolysis has much increased acid value and very low compatibility with the plasticizer to cause considerable deterioration of film-formability. Moreover, since such shell portion with high acid value contributes for polymer particles in the plastisol to form structural viscosity, there is the problem of deterioration in operability due to an increased viscosity of the plastisol.
Furthermore, JP-A-53-144950 proposes another example of plastisol using particles of core-shell structure. It employs a method of obtaining core-shell structure by the stepwise polymerization of monomers having different compositions. In this patent publication, a shell incompatible with plasticizer is used for developing storage stability of the plastisol and there is used a shell formed by copolymerization of at least 80% by weight of methyl methacrylate having low compatibility with many plasticizers. The shell having very low compatibility is advantageous for storage stability, nevertheless, has the tendency of becoming inferior in various performances such as film-formability of sol, and strength, elongation, transparency, adhesion to substrate, sound insulation and vibration-damping properties of the resulting coating film. Especially, since the plastisol is inferior in retention of plasticizers, it tend to cause bleed out of the plasticizers and thus not practical.
Further examples of plastisols prepared using particles of core-shell structure are disclosed in JP-A-7-233299 and JP-A-8-295850. In these examples, fundamental performances are achieved by using core-shell polymers comprising a core portion showing basic compatibility with plasticizers and a shell portion showing basic incompatibility with plasticizers. However, since extremely excellent properties of a polymer are demanded for putting it to practical use, the polymers proposed in the above patent publications, in this respect, are not optimized in balancing of compatibility with plasticizers and are at low levels in both the storage stability and the plasticizer retainability and, therefore, unsuitable for industrial practical use.
As mentioned above, though various researches have been made on acrylic sols to make compatible the storage stability and the plasticizer retainability which are the most basic properties of plastisols, they are all at too low levels as substitutes for vinyl chloride sols to reach industrially practical levels.